Field of the Invention
The present invention relates to a crystalline phase identification method, a crystalline phase identification device, and an X-ray diffraction measurement system for identifying a crystalline phase contained in a sample composed of a powdery crystal or a polycrystalline substance from X-ray diffraction data of the sample.
Description of the Related Art
Most solid substances exist in a crystalline state. Many solid substances are made of aggregated fine crystal grains. Aggregated fine crystal grains are referred to as a polycrystalline substance. X-ray diffraction measurement for handling powdery crystals or polycrystalline substance as a sample is called X-ray powder diffractometry.
An X-ray diffraction pattern of a sample obtained by measurement using an X-ray diffractometer is characteristic for each crystalline phase. A crystalline phase contained in a sample can be identified by analysis of the X-ray diffraction pattern. As used herein, the term ‘crystalline phase’ refers to the crystal structure and chemical composition of a substance which is crystalline. JP-A 2014-178203 discloses a technique for carrying out a qualitative analysis for identifying a crystalline phase contained in a sample with good precision on the basis of the powder diffraction pattern of a sample.
When the number of crystal grains in a sample is sufficiently large and the directions of lattice planes are random, a lattice plane having an angle that satisfies diffraction conditions must be present. X-rays diffracted by a lattice plane at a diffraction angle of 2θ proceed along the generating line of a cone in which the half apex angle is 2θ when 2θ<90°, and proceed along the generating line of a cone in which the half apex angle is (180°−2θ) when 2θ>90°. In other words, X-rays diffracted by a sample composed of powdery crystal or a polycrystalline substance form numerous cones having different central angles. When such X-rays are received in the detection surface of an X-ray detector, a concentric circular diffraction pattern is obtained. This diffraction pattern is referred to as Debye-Scherrer rings.
Debye-Scherrer rings obtained by X-ray powder diffractometry include a plurality of rings. Uniformity in the circumferential direction of the rings (i.e., the diffraction pattern) reflects the state of particles contained in a sample. When there are lattice planes that yield a diffraction pattern in which intensity is uniform in the circumferential direction and lattice planes that yield a diffraction pattern in which intensity is not uniform in the circumferential direction, the states of grains that contain these lattice planes are different from each other.
Qualitative analysis is an example of an analytic method for identifying a crystalline phase. In a qualitative analysis, the two-dimensional data of an X-ray diffraction pattern of a sample is converted to “diffraction angle 2θ versus intensity I data” to create a “diffraction angle 2θ versus intensity I profile.” The position and intensity of peaks in the “profile of the diffraction angle 2θ versus intensity I profile” are detected.
“Diffraction angle 2θ versus intensity I data” may hereinafter be referred to as “2θ-I data.” The “diffraction angle 2θ versus intensity I profile” may hereinafter be referred to as the “2θ-I profile.”
A system for carrying out a qualitative analysis has a database. “Peak position” data and data for the “ratio of the peak intensity between a plurality of diffraction patterns” in an X-ray diffraction pattern are registered in the database. The “ratio of the peak intensity between a plurality of diffraction patterns” may hereinafter be referred to as “peak intensity ratio.”
A system for carrying out a qualitative analysis performs a search in accordance with installed software. Specifically, data related to a plurality of known crystalline phases registered in the database is searched, and candidates of crystalline phases contained in a sample are extracted on the basis of the position of peaks detected from the sample and the peak intensity ratio detected from the sample, like the position of peaks. The search using the database in this case may be referred to as “search and match.”
Conventionally, the uniformity of diffraction patterns in the circumferential direction is not taken into consideration when search conditions are set by software during a search and match. Accordingly, it may be possible for unintended crystalline phase candidates to be listed in the search results on the basis of diffraction pattern groups that are not attributable to the same crystalline phase, and, consequently, diffraction pattern groups having different uniformities in the circumferential direction.